Using high temperature thermocouples in vacuum furnaces

Temperature measurement in vacuum furnaces requires thermocouples that remain stable at extreme temperatures while resisting contamination, outgassing and metallurgical degradation.

These demanding conditions create specific challenges for sensor selection and performance. This article introduces vacuum furnace environments and explains which thermocouples are best suited for reliable operation.

What is a vacuum furnace?

A vacuum furnace is a special type of furnace that is capable of heating materials, especially metals, to extremely high temperatures. What makes it unique is that it operates in a vacuum, which means there is no air or other gasses present inside the furnace during the heating process, which serves several important purposes.

• Firstly, it prevents the materials from reacting with oxygen in the air, which can be damaging to certain substances.
• Secondly, the vacuum environment helps to reduce heat loss from the materials due to convection, whereby heat is transferred away by the movement of air or gas.
• Lastly, it ensures that there are no potential sources of contamination present inside the furnace.

Applications of vacuum furnaces

Thanks to these properties, a vacuum furnace can achieve exceptionally high temperatures, reaching up to 3000 °C (5432°F). This makes it suitable for various applications that require exposing materials to extreme temperatures with high consistency and low contamination, such as sintering and heat treatment.. Additionally, the vacuum furnace can be utilized for vacuum brazing. Brazing is a method used to join metal parts together. In this process, a special metal, known as filler metal, is heated until it becomes liquid. This liquid filler metal is then drawn into the small gaps between the two or more metal parts being joined, thanks to capillary action.

To protect the filler metal and ensure a successful brazing process, it is heated slightly above its melting temperature (known as the liquidus temperature), while being exposed to a controlled atmosphere with lower pressure than the standard atmospheric pressure. This controlled atmosphere is typically a vacuum.

Once the liquid filler metal has flowed over the surface of the metal parts being joined (a process known as wetting), the whole assembly is cooled down. As a result, the filler metal solidifies, firmly joining the metal pieces together. This vacuum brazing technique is used to create strong and precise connections between various metal components in different industries. One example is our mineral insulated heating elements Temperature uniformity can be achieved by vacuum brazing these mineral insulated heating elements into grooves on stainless steel plates.

Which thermocouples are suitable for vacuum furnaces?

Due to the broad temperature range of vacuum furnaces, several different types of thermocouples are typically used for this application. The thermocouple types include:

• Type K thermocouples (-200 °C to 1250 °C) are one of the most widely used temperature sensors due to their wide temperature range and good accuracy. They are suitable for many high-temperature applications including vacuum furnaces. Generally, these thermocouples are used in MI cable form for temperatures up to 1150°C. Above these temperatures, non-metal tubes, such as C610 tubes, are used for protection.
• Type N thermocouples have a temperature range similar to Type K (-200 °C to 1300 °C) but offer better accuracy and stability at high temperatures. As with Type K thermocouples, Type N are used in MI cable form for temperatures up to 1150°C. Above these temperatures, non-metal tubes, such as C610 tubes, are used for protection.
• Type S, R, and B Thermocouples: These are noble metal thermocouples with high accuracy and stability at very high temperatures. Type S thermocouples cover a temperature range of 0 °C to 1600 °C, Type R thermocouples from 0 °C to 1600 °C, and Type B thermocouples from 600 °C to 1800 °C. Generally, these thermocouples are only used in vacuum for short term or in combination with a protective sheath.
• Type C and D Thermocouples: Type C (also known as Type W) and Type D thermocouples are suitable for ultra-high temperature applications. Type C covers a range of 0 °C to 2300 °C, while Type D covers a range of 0 °C to 2300 °C as well but is designed for use in vacuum or inert atmospheres. Generally, these thermocouples are used in combination with molybdenum protection tubes, as well as tantalum protection tubes.

C799, Molybdenum or tantalum protection tubes

Molybdenum has a significantly higher melting point (2610°C) compared to the maximum operating temperature of C799 (1800°C). Molybdenum has excellent resistance to high temperatures and extreme thermal environments. It remains stable and does not readily react with other materials at temperatures commonly found in vacuum furnaces. Furthermore, Molybdenum has higher mechanical strength and toughness compared to C799 protection tubes, making it more durable and resistant to potential mechanical stresses or damage within the vacuum furnace environment.

Kamet’s thermocouples for vacuum furnace applications

Thermocouples used in high-temperature environments are essential in industries where demanding conditions exist, such as in special vacuum furnaces. While regular thermocouples (type K or N) or thermocouples made of platinum have some limitations, type C and D thermocouples are specially designed to work better in very high temperatures and challenging situations that platinum thermocouples cannot handle. These ultra high thermocouples provide better accuracy and reliability in extreme conditions.